Engines may be configured with direct fuel injectors that inject fuel directly into a combustion cylinder (direct injection), and/or with port fuel injectors that inject fuel into a cylinder port (port fuel injection). Direct injection allows higher fuel efficiency and higher power output to be achieved in addition to better enabling the charge cooling effect of the injected fuel.
Further, during engine cold-starts, direct injection of fuel during a power stroke or exhaust stroke (also known as a post fuel injection) or late in a compression stroke allows for expedited heating of an exhaust catalyst. One example approach for expediting exhaust catalyst heating is shown by Nagai et al. in U.S. Pat. No. 6,374,798. Therein, fuel is injected in a compression stroke when more catalyst heating is required, and in an intake stroke when less catalyst heating is required.
However, the inventors herein have realized that gasoline direct injected engines generate more particulate matter emissions (or soot) during cold-starts and engine warm-up due to diffuse flame propagation wherein fuel may not adequately mix with air prior to combustion, as well as due to cylinder wall wetting. Since direct injection, by nature, is a relatively late fuel injection, there may be insufficient time for mixing of the injected fuel with air in the cylinder. Similarly, the injected fuel may encounter less turbulence when flowing through the valves. Consequently, there may be pockets of rich combustion that may generate soot locally, degrading exhaust emissions. Likewise, delivery of gasoline as a post fuel injection or a late compression stroke injection via a direct injector can lead to increased piston fuel wetting and a significant increase in tailpipe particulate emissions.
The inventors herein have recognized that at least some of the above mentioned issues may be addressed using methods for an engine system operating with direct injection of a gaseous fuel, such as liquefied petroleum gas. One example method comprises: during an engine cold-start, combusting a first amount of gaseous fuel during one or more of an intake stroke and a compression stroke of a first combustion event; and combusting a second amount of gaseous fuel during a power stroke of the first combustion event, a ratio of the first amount to the second amount adjusted to enable a rich air-fuel ratio at the spark plug for improved engine stability while maintaining overall combustion air-fuel ratio at stoichiometry in the cylinder. In this way, catalyst heating can be expedited without degrading exhaust emissions.
As an example, an engine system may be configured with a liquefied petroleum gas (LPG) fuel delivery system and the gaseous fuel (e.g., LPG) may be direct injected into the combustion chamber. During an engine cold-start condition, such as when an exhaust catalyst temperature is below a threshold temperature or efficiency, the gaseous fuel may be delivered to the engine as one or more of an intake stroke injection and a compression stroke injection. The fuel injection may be biased more towards the compression stroke injection as the exhaust catalyst temperature at the engine cold-start decreases. As such, the intake stroke injection may enable good mixing and during catalyst heating the injection may be lean. The compression stroke injection may then be used so that the air-fuel ratio at the spark plug is near stoichiometric so that the mixture ignites easier. Optionally, fuel may also be injected as single or multiple injections during the power stroke and combusted in the exhaust port. The resulting increase in exhaust temperature and pressure reduces the time till catalyst light-off. An amount of fuel injected in the intake, compression, and power strokes may be adjusted so as to maintain an overall exhaust air-fuel ratio at or around stoichiometry. In addition, a timing of the injections may be adjusted based on the catalyst temperature and spark timing. For example, as the catalyst temperature at the cold-start conditions decreases, the compression stroke injection may be performed closer to compression stroke top dead center (TDC) while the power stroke injection(s) is performed further from TDC. As another example, a smaller portion of fuel may be injected in the intake stroke while a larger portion of fuel is delivered in the compression stroke and as a post injection (in the power stroke). The split fuel injection strategy may be continued as the catalyst temperature or efficiency increases. When the exhaust catalyst is sufficiently warm (e.g., is at or above the light-off temperature), the post injection may be discontinued and fuel injection in one of the intake stroke or the compression stroke may be resumed. Alternatively, the split fuel injection strategy may be modified to discontinue the post fuel injection (in the power stroke) while maintaining the intake and compression stroke fuel injections until the catalyst is lit-off.
In this way, by injecting fuel into a cylinder after compression stroke TDC, the increased oxidation of hydrocarbons and carbon monoxide further increases exhaust temperature while reducing feedgas emissions. Overall, catalyst light-off efficiency is improved without degrading exhaust particulate
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.